This invention relates to apparatus for removing impurities from liquids by reverse osmosis, and relates more particularly to an element configuration and method of manufacture thereof, and to a cartridge unit for mounting reverse osmosis purification elements.
In reverse osmosis purification systems, a pressurized fluid containing one or more undesired components is applied to the surface of a semipermeable membrane which results in a solvent portion of the fluid passing through the semipermeable membrane and a preponderance of the undesired components becoming more concentrated at the membrane surface. Reverse osmosis systems have been used for many purposes, including the provision of relatively small amounts of purified water for home usage. These systems have frequently employed one or more suitably housed membrane elements which are continuously contacted with mineral-containing water. Purified water (which may be thought of as a solvent) slowly permeates through the membrane and is collected in a storage tank, such as a bladder tank, for use on demand.
A variety of different supports for the semipermeable membrane and housings for the membrane elements have been used. Membrane elements frequently comprise a rigid tubular member around which is wrapped a spacing screen. A thin semipermeable membrane is then formed over the screen by casting or dipping the tubing into a solution of membrane-forming liquid, such as a cellulose acetate solution. Water passes through the cellulose acetate membrane and the screen, entering a center portion of the support tube, e.g., through perforations along the tube; pure water exits one end of the tube. Various methods and apparatus for applying semipermeable membrane layers to tubular units are known and form no part of the present invention. Examples of such method are found in Thayer et al, U.S. Pat. Nos. 3,929,945, and 4,177,031. Similarly, any semipermeable membrane film may be used in the apparatus of the invention, including the cellulose acetate films disclosed in Loeb et al, U.S. Pat. No. 3,133,132, and Manjikian, U.S. Pat. No. 3,344,214.
The particular invention is concerned first with a specific tubular semipermeable membrane element and a method of making the same, and secondly with a particular cartridge configuration for housing a plurality of tubular membrane elements. The elements consist of an elongate rigid tube member having a plurality of longitudinal channels or grooves extending along the entire length of the tube. The tube has a monofilament helically wrapped with about 0.5 mil spacing around its exterior surface. A semipermeable membrane film is mounted on the monofilament wrapping. The membrane is applied to the monofilament surface by applying a liquid solution of membrane-forming materials thereto in any conventional manner, e.g. by extrusion, casting, or dipping, and permitting the solvent to evaporate to leave a thin, rigid, relatively uniform film. The membrane support is manufactured by cutting a plurality of V-shaped grooves into the tube, and wrapping monofilament around the tube, e.g. on a lathe.
In the past, membranes have been cast or otherwise fixed over a base generally comprising a fabric or fibrous material. Because the base material has a plurality of "ends" which may extend randomly through the membrane after casting, a relatively high failure rate caused by imperfections or "holidays" in the membrane surface has been realized. The monofilament base of the present invention has led to a failure rate of elements well below that heretofore realized.
The membrane elements of the invention are particularly adapted for use in relatively small cartridges which are suited for home use. The housing is designed to provide as much membrane surface area as is practical for each unit of volume of the housing; this permits a relatively small unit which may be placed, for example, under a kitchen sink, but which still provides relatively high throughput. In addition, the housing design is particularly adapted to provide internal flow of impure water around the membrane elements to insute that impurities are swept away from the surface efficiently. The housing also includes a safety system to prevent the possibility of back pressure from a pressurized purified water tank from damaging the membrane in the event of a failure of line pressure of the impure water source. Since membrane elements must be replaced after certain periods of use, the cartridge of the invention is also designed for easy installation and replacement.
Accordingly, it is an object of the invention to provide a reverse osmosis membrane element which is easy to manufacture, has a relatively high throughput, and is subject to a relatively low failure rate. It is another object of the invention to provide a housing for mounting a plurality of membrane elements which is inexpensive, easy to assemble, and which protects the membrane elements from damage in the event of loss of line pressure in the water supply system. It is yet a further object of the invention to provide a housing which directs flow of impure liquid around the membrane elements in a manner of flush away impurities accumulating at the surface thereof. These and other objects of the invention will be apparent from the following detailed description of an embodiment thereof.